Cellulose bleaching plant

ABSTRACT

The present invention concerns a cellulose bleaching plant. A container forming part of said plant is provided for continuous axial flow of the pulp to be bleached and is provided with means for exchanging the liquid contents of the pulp. Said means comprise hollow screen bodies and movable spray tubes. Liquid supplied through said tubes and distributed into the pulp displaces the previous liquid contents of the pulp, such as a bleaching solution, which enters the screen bodies and is then discharged. The cavities of the screen bodies are divided into height zones having separate outlets, whereby the discharged liquid is fractionated. The spray tubes supply different liquids in corresponding height zones of the container. Thus a double exchange of liquids is made possible, and the washing liquid that has replaced the bleaching liquid may in its turn be replaced by another chemical treatment liquid.

United States Patent Richter ct al., ..68/18l R Richter Dec. 5, 1972 [541 CELLULOSE BLEACHING PLANT 3,348,390 10 1967 Richter .Q ..68/18l R Inventor: Johan C. F- C- Richter St. Jean Rlchter X Ferrat France 1 Q Primary Examiner-Walter A. Scheel [73] Assignee: Aktiebolaget Kamyr, Karlstad, Assistant Examiner-Philip R. Coe Sweden AttorneyCushm'an, Darby & Cushman {22] F1led: Oct. 30, 1970 [57] ABSTRACT [2]] App! 85;, The present invention concerns a cellulose bleaching Remed Appficafion Data plant. A container forming part of said plant is provided for continuous axial flow of the pulp to be commuatm" 628,417 P" 41 1967" bleached and is provided with means for exchanging abandonedthe liquid contents of the pulp. Said means comprise hollow screen bodies and movable spray tubes. Liquid Foreign Appllcallon Prlonty Data supplied through said tubes and distributed into the Oct. 27 1966 Sweden ..14748/66 P displaces the Previous liquid 0f the Pu such as a bleaching solution, which enters the screen [52] u.s.c1 ..68/l8 F, 68/181 R, 162/248, bOdieS and is discharged- The cavities of the 210/203 210/327 screen bodies are divided into height zones having [51 Int. Cl. ..D21d s/02,-1$2le 9/18 Separate ("meme whereby the discharged liquid is frac- [58] Field of Search ..8/l56; 68/18 F, 181 R; 162/19, 3. if? tubes w g correspon lng erg t zones 0 t e container. us a 210/203 7 double exchange of liquids is made possible, and the washing liquid that has replaced the bleaching liquid [56] References Cited may in its turn be replaced by another chemical treat- UNITED STATES PATENTS ment q 3,078,703 2/1963 6 Claims-3 Drawing Figures PUMP MEANS PKTENTEMEc 1912 3 704603 sum 1 [1F 3 PATENTEDBEB m 3.704.603 SHEET 3 BF 3 INVEN TOR (fai /4w 67 61/50/22? ATTORNEYs CELLULOSE BLEACHING PLANT This is a continuation of application Ser. No. 628,417 filed Apr. 4, l967, and now abandoned.

The present invention relates to a cellulose bleaching plant comprising an upright cylindrical container provide with means for continuously feeding cellulosic pulp axially therethrough, further provided with a screening device located within the container andcomprising a set of concentric hollow screen bodies located at the same level and each having cylindrical screen faces and an interior cavity for screening-off and discharging part of the liquid contents of the pulp, said container also having movable spray tubes for the supply and distribution of liquid into the pulp. The container with screening device and spray tubes may be of the general type shown and described in the US. Pat. specification No. 3,078,703.

The object of the invention is to make the screening and spraying devices of the container suitable for performing therein of several of the different operations required in connection with bleaching of the pulp, thereby decreasing the number of components of the bleaching plant.

According to the invention said object is accomplished by dividing up the screen bodies unto height zones having separate outlets, and the spray tubes connected to conduits for supply of different liquids have their orifices located at different levels. In this way two different liquids can be distributed in the container and also two separate liquids can be screened-off and discharged therefrom, the container being adapted to permit a double exchange of liquids wherein liquid supplied in order to displace at least part of the original liquid contents of the pulp in its turn is displaced at least partly by a second supplied liquid and is collected separately, i.e., without being mixed with the first displaced liquid. Thus, the reaction products resulting, e.g., from a bleaching reaction in the container, together with non-spent treating chemicals, if any, can be displaced by a washing liquid, such as more or less pure water, and said washing liquid can immediately thereupon be displaced by a liquid containing the bleaching chemicals required for performing a subsequent bleaching step.

The invention also relates to a method of bleaching cellulosic pulp, which method can be practised in the above described apparatus and which consists in that alkali is supplied through spray tubes having their orifices at an intermediate level, whereas washing liquid is supplied through spray tubes having their orifices at a lower as well as at a higher level, so that an alkali treatment step is performed at the level of the screen bodies in a container also used for performing a preceding bleaching step, e.g., with chlorine or chlorine dioxide.

The invention will be more closely described herein below with reference to accompanying drawings, in which FIG. 1 is a vertical sectional view of the upper part of a bleaching tower according to the invention,

FIG. 2 diagrammatically shows a part thereof on an enlarged scale and the various flows of liquid present therein when performing the above-mentionedalkali treatment step and FIG. 3,is a plan view of the apparatus of FIG. 1 with parts removed for clarity.

The bleaching tower shown in FIG. 1 forms part of a plant for bleaching of cellulosic pulp, consisting of a number of towers connected in series, through which the pulp is fed continuously by, for example, pump means as diagrammatically shown in FIG. 1 and in which a succession of different treatment steps required for the bleaching of the pulp are performed. Thus, the tower shown may be preceded and/or followed by one or more towers of an essentially similar design.

By means of a pump, pulp is supplied continuously to the lower end (not shown) of the tower and is spread there (e.g. by means of a spreading device of the design shown in the US. Pat. specification No. 2,764,01 1) over the cross-sectional area of the tower, so that the pulp as indicated by the arrows 1 l in FIG. 1 moves axially upwards through the tower with substantially the same speedeverywhere. At its upper end the eylindri cal shell 13 of the tower has a widened part 15 with an outlet 17, into which the uppermost layer of the pulp column slowly rising in the tower is diverted by means of a discharging device consisting of scraper blades 19 attached to arms 21 which extend radially from a shaft 23 concentric to the tower shell and driven by'a motor 22.

A screening device arranged in the tower is composed of screen bodies 25, 27, 28, concentric to each other and to the tower shell 13. The screen body 25 has an inner screen face and an outer screen face in the shape of a perforated or slitted cylindrical plate 29 or 31, respectively, and an inner cavity. The same'applies to the screen body 27 whereas the central screen body 28 has a single cylindrical scr-en plate. The screen bodies are located at the same level and are of the same axial extension, and they are mutually connected and supported by horizontally and radially directed carrying arms 33, the cross-sections of which preferably are streamlined in respect of the axial flow of the pulp. Said arms extend out through the shell 13, and a hydraulic operating device 35 is attached to the outer end of each of them. The described set of screen bodies is somewhat displaceable in the axial direction in the tower, whereby it will accompany the slowly rising pulp a short distance upwards, and by the operating devices 35 the screen bodies are periodically and comparatively rapidly returned in the downward direction, a relative motion taking place between their screening faces and the pulp.

According to the invention the screen bodies 25, 27, 28 are divided into height zones having separate outlets. Thus, the interior of the screen body 28 is by means of horizontal partitions 37, 39 divided into three separate cavities 41, 43, 45 which are connected each to a separate passage 47, 49 and 51, respectively, in the carrying arms 33. In their turn said passages are connected outside the tower shell to three separate exterior outlet conduits 53, 55 and 57, respectively. The interior of the screen bodies 25 and 27 is divided in a similar manner, and their cavities 59, 61, 63 and 60, 62, 64 which are located at the same level and have the same axial extent as the cavities 41, 43 and 45, respectively, are also connected to the passages 47, 49, 51, respectively, and to the discharge conduits 53, 55, 57, respectively. The liquid fractions screened-off the pulp by the screen bodies at their various height zones, thus are held separated from each other and are discharged from the tower each per se.

In order to replace, wholly or partly, the liquid screened-off the pulp by the screen bodies there is provided a device for supply of replacement liquid and the distribution thereof within thepulp. According to the invention said device is divided into two separate systems and two dissimilar liquids can be distributed at different levels in the tower.'One system comprises a stationary supply conduit 65 with control valve 67, a tank 69 attached to the shaft 23, distribution tubes 71 and vertical spray tubes 75, 77, 79 having outlet orifices 81, 83, 85, respectively. By means of adjustable gates 86 the liquid quantity supplied to the tank 69 is distributed in proper proportions upon three compartments 88 located outside of the tank 69 and consequently'upon said three outlet orifices. The other liquid supply system comprises a stationarysupply conduit 87 with controlvalve 89, a revolving tank 91, three distribution compartments 93, distribution tubes 95, and vertical spray tubes 99, 101, 103 provided with upper outlet orifices 105, 107, 109 and lower outlet orifices 111, 113, 115. The outlet orifices are prolonged in the vertical direction so that when the shaft 23 is turned, each outlet orifice willspread liquid over a cylindrical face. The orifices 81, 8,3, 85 have the same vertical extension and are located, at the same level, and the same applies to the orifices belonging to the group 105, 107, 109 and' the group 111, 113, 115. The spray tube 75 belonging to the first liquid distribution system moves in a path having thesame radius as the path of the spray tube 99 belonging to the other liquid distribution system, but the outlet orifice 81 of the former is axially off-set relatively to the orifices 105 and 111 of the last-mentioned spray tube. Therefore, liquid of the kind supplied through the conduit 65 is distributed over a cylindrical face having the same radius as, but an other location in the height direction than the cylindrical faces over which liquid from the conduit 87 is spread. The same applies to the relative position of the orifices 83, 107, 113 and to the relative position of the outlet orifices 85, 109, 115. Thus, the entire liquid distribution device spreadstwo liquids over different height zones, and in each zone the liquid is distributed upon outlet orifices moving between or on one side only of the screen bodies 25,27, 28. Furthermore, the outlet orifices are located in the. level range of the screen bodies, involving that the displacement flow of liquid through the pulp will be transversally directed relatively to the axial flow of the pulp,

, viz. substantially horizontal and radial. The three levels of the outlet orifices may correspond approximately to the three levels of the divisions of the screen bodies, but often a certain mutual offset in the axial direction is preferable. For instance, the orifices 81, 83, 85 may be placed opposite to the lower portion of, or even partly lower than the center cavities 43, 61, 62 of the screen bodies.

The described plant is suitable among others for performing of a cellulose bleaching operation in two steps, viz. a first step consisting of the treatment of the cellulosicpulp with chlorine or chlorine dioxide, performed level than the level where the alkali solution is spread.

in the lower part of the tower, and a second step consisting of the treatment of the pulp with alkali performed in the top end of the tower and at the level of the screen bodies. In this case a solution of e.g. sodium hydroxide is supplied through'the conduit and is For pulp thathas risen'to the level of the lowermost portion of the screen bodies, the chlorine bleaching is completed and the soluble reaction products resulting therefrom should be removed. Therefore, they are displaced by washing water that is spread from the'lowermost series of outlet orifices 111, 1 13, 115 and forces a corresponding quantity of liquid out through the lowermost zone of the screen bodies and through the conduit 57 to the drain. The alkali solution spread at the intermediate level through'the outlet orifices 81, 83, in its turn displaces the washing water and forces the same out through the middle zone of the screen bodies and further on to the discharge conduit 55. Finally the washing water supplied through the upper spray tube outlets 105, 107, 109 displaces the alkali solution out through the uppermost zone of the screen bodies and further on to the outlet conduit 53, so that the pulp leaving the tower through the outlet 17 is washed sufficiently clean to be mixed with new bleaching chemicals for the performance of a subsequent bleaching step in an other tower.

As shown inFIG. 2, on account of the axially upwardly directed movement of the pulp and the radially inwardly directed flow of liquid, conical border faces are formed between pulp portions containing different liquids. As these border faces are not sharp but mixed layers are formed, the quantities and flow rates of the liquids supplied through the spray tubes should be adjusted in such a manner that a certain overdi splacement takes place, whereby washing liquid may enter the lowermost as well as the intermediate cavities of the screen bodies. However, the alkali solution is screenedoff completely or almost completely through the middle zone of the screen bodies. It may be suitable to branch-off a conduit 117 from the conduit 55 carryingoff alkali in order to hold part of said alkali in circulation by mixing it with the newly added alkali supplied through the conduit 65, the returned alkali thus being supplied to the spray tubes having their outlets at the intermediate level. The rest of the screened-off alkali is carried through the conduit 119 either to a preceding alkali treatment step of the bleach plant or to an evaporation plant for the recovery of the alkali or possible to a digester as admixture to the digesting liquor used therein. 9

The above described embodiment forms merely an example which may be modified in various respects within the scope of the invention. Thus, a division by three may apply also to the liquid supply, separate supply conduits being provided and connected to the upper and the lower outlet orifices, as shown in FIG. 2, where the conduit 121 is not connected to the conduit 87 but may, for instance, be connected to the discharge conduit 53, whereby the washing water flowing in the latter and containing merely traces of alkali is returned and used in the first washing step at the level of the lowermost zone of the screen bodies.

An other. modification may consist in restricting the number of zones of the screen bodies as well as of the liquid distribution system. In this case the uppermost washing step in FIG. 2 is dropped, and the screening bodies at the top of the tower are used merely for the removal of reaction products resulting from the bleaching step performed in the main part of the tower and for the impregnation of the pulp with such chemicals as are required for the subsequent bleaching step which takes place in an other tower. Said step may be an alkali treatment, but preferably some other treatment requiring a longer reaction time and therefore not performable in the top end of the tower of FIG. 1.

FIG. 3 herein illustrates how liquid in the various height zones is removed and discharged through the passages 47, 49 and 51 and their associated outlet conduits 53, 55 and 57, respectively. As shown in FIG. 1, liquid in the top height zone. 41 of thescreen body 28 passes downwardly through the central passage 42 for discharge by way of passage 47 and outlet'53. The top cavities 59 and 60 of screen bodies 25 and 27, respectively, are likewise joined to passage 47 and outlet 53 by vertical conduits 42 a and 42 bvwhich empty into passage (47) as best shown in FIG. 3. The middle cavity 43 of screen body 28 empties into passage 49 by way of the vertical conduit 44 (see FIG. 1) and similarly the middle cavities 61 and 62 of screen bodies 25 and 27 empty into passage 49 by means of vertical conduits 44 a and 44 b, respectively as shown in FIG. 3.

The bottom cavities or height zones 63 and 64 of bodies 25 and 27 empty into passageway 51 by connection through the V-shaped bottom portions of the screen bodies as shown by the numeral 46 in FIG. 1.

We claim:

1. A cellulose bleaching vessel for cellulose pulp comprising an upright cylindrical container through which said pulp is axially passed, said container being provided with a screening device located within the container and comprising a set of concentric hollow screen bodies of different diameter and whose centers are aligned with the longitudinal axis of said container, said bodies being located at the same level and each having cylindrical screen faces and an inner cavity for horizontally screening off part of the liquid contents of the pulp through said screen faces and into the inner cavity for discharge as said pulp passes said screen bodies, said container also having at least two movable spray tubes adjacent said screen bodies and including means for supply and distribution of liquid into the pulp, the inner cavity of each of said screen bodies being divided up by means of horizontal partitions into a plurality of different height zones with corresponding height zones in each of said screen bodies and separate liquid discharge outlets for each said height zone, said spray tubes being connected to conduits for supply of different liquids and having liquid discharge orifices located at different levels with respect to the height zones of said screen bodies so that each of said different liquids is sprayed into said pulp at a different level and liquid is screened off at one height zone and discharged separately from liquid screened off at a different height zone through said separate liquid discharge outlets.

2. A vessel as claimed in claim 1, characterized in that the interior of each screen body located in the same height zone is connected to the same discharge 0 tlet.

3. A vessel as claimed in claim 1, characterized in that the discharge outlets from different height zones of the screen bodies comprise parallel passages within radial arms carrying the screen bodies.

4. A vessel as claimed in claim 1, wherein spray tubes connected to different liquid supply conduits are positioned so as to spread the liquid discharged therefrom over coaxial cylindrical surfaces having the same radius located at different levels.

5. A vessel as claimed in claim 1, characterized in that the discharge means of the spray tubes are distributed over height zones of essentially the same location as the height zones of the screen bodies, whereby the direction of the liquid flow through the pulp predominantly is substantially horizontal and radial.

6. A vessel as claimed in claim 1, wherein there is branched-off from one of the discharge outlets of the screen bodies a conduit which is connected to and serves for supplying screened-off liquid to spray tubes having orifices located at the same level as, or referred to the movement of the pulp upstream of, the zone of the screen bodies connected to said discharge outlet. 

1. A cellulose bleaching vessel for cellulose pulp comprising an upright cylindrical container through which said pulp is axially passed, said container being provided with a screening device located within the container and comprising a set of concentric hollow screen bodies of different diameter and whose centers are aligned with the longitudinal axis of said container, said bodies being located at the same level and each having cylindrical screen faces and an inner cavity for horizontally screening off part of the liquid contents of the pulp through said screen faces and into the inner cavity for discharge as said pulp passes said screen bodies, said container also having at least two movable spray tubes adjacent said screen bodies and including means for supply and distribution of liquid into the pulp, the inner cavity of each of said screen bodies being divided up by means of horizontal partitions into a plurality of different height zones with corresponding height zones in each of said screen bodies and separate liquid discharge outlets for each said height zone, said spray tubes being connected to conduits for supply of different liquids and having liquid discharge orifices located at different levels with respect to the height zones of said screen bodies so that each of said different liquids is sprayed into said pulp at a different level and liquid is screened off at one height zone and discharged separately from liquid screened off at a different height zone through said separate liquid discharge outlets.
 2. A vessel as claimed in claim 1, characterized in that the interior of each screen body located in the same height zone is connected to the same discharge outlet.
 3. A vessel as claimed in claim 1, characterized in that the discharge outlets from different height zones of the screen bodies comprise parallel passages within radial arms carrying the screen bodies.
 4. A vessel as claimed in claim 1, wherein spray tubes connected to different liquid supply conduits are positioned so as to spread the liquid discharged therefrom over coaxial cylindrical surfaces having the same radius located at different levels.
 5. A vessel as claimed in claim 1, characterized in that the discharge means of the spray tubes are distributed over height zones of essentially the same location as the height zones of the screen bodies, whereby the direction of the liquid flow through the pulp predominantly is substantially horizontal and radial.
 6. A vessel as claimed in claim 1, wherein there is branched-off from one of the discharge outlets of the screen bodies a conduit which is connected to and serves for supplying screened-off liquid to spray tubes having orifices located at the same level as, or referred to the movement of the pulp upstream of, the zone of the screen bodies connected to said discharge outlet. 